This invention relates to a light beam scanning apparatus such as an image recording apparatus. More particularly, this invention relates to a light beam scanning apparatus capable of recording an image of high quality with the density of light beam scanning lines being changed by altering sub-scanning speed.
Light beam scanning apparatuses using a raster scanning method are applied in various image recording apparatuses. With such light beam scanning apparatuses, a light beam reflected and deflected in the direction of main scanning by means of a light deflector such as a galvanometer mirror, is applied at a predetermined position onto a sheet-like subject, such as a recording material, to be scanned that is transported in a sub-scanning direction that is generally perpendicular to the main scanning direction. Hence, in light beam scanning apparatuses that adopt a raster scanning method, the sheet-like subject is scanned two-dimensionally with a light beam to effect image recording, or image reading.
As improvements of light beam scanning apparatuses that adopt a raster scanning method, there have been proposed and used commercially those apparatuses in which the transport speed in the sub-scanning direction is reduced so that the scanning density is increased to enable the recording of a higher-quality image. Instead of changing the time interval between successive main scanning cycles, the speed at which the sheet-like subject is transported in the sub-scanning direction is made slower than in the ordinary image recording mode, where by the interval between scanning lines on the sheet-like subject is reduced to enable the production of a high-quality image at an increased recording density.
With such light beam scanning apparatuses that are adapted to change the density of scanning lines, the intensity of the light beam applied to the sheet-like subject must be reduced in proportion to the increase in scanning density. If the intensity of light beam applied remains unchanged despite the increase in scanning density, overexposure takes place in the image recording apparatus and line in characters or line images will become so thick that "blocking of shadows" and other defects will result and preclude the recording of a satisfactory image.
When the scanning density is to be increased by slowing down the transport speed in the sub-scanning direction, the intensity of the light beam applied is conventionally reduced by inserting a ND (neutral density) filter or some other means of adjusting the intensity of the light into the path of the light beam. However, in order to implement this method, not only is a filter inserting mechanism necessary but also the space for permitting the insertion of the filter is necessary. The light beam scanning apparatus will inevitably become bulky and complicated in construction, with its cost increasing. Further, the light beam passing through the ND (neutral density) filter will experience wavefront aberrations, so in order to achieve satisfactory image recording, high-quality filters that have a surface with a high precision of flatness are required but, then, this results in considerable increase in the cost of the light beam scanning apparatus.